How to mix two baked normal maps on one mesh
You will need to select which UV map is used for which normal map. Go to the mapping panel in the texture tab for each normal map and make sure mapping is set to UV and the appropriate UV map is selected.

Also, when you create a UV map, every face from your object is on every map, so you will need to make sure that the face UVs are on an area of the Body UV map where the texture is transparent and vice versa. If you select all of the faces of your mesh in edit mode and look at each map you will notice that the UVs for the face and body are on each map. Simply select all of the islands for the face and scale them down until they fit on one of the transparent sections on the Body Normals image, and do the same to the Face Normals map but scale down the body island instead.

Also in your case you'll want to turn down the strength of the normals to .1-.2 or something as otherwise they make your mesh quite grey. Your lighting setup is also causing a few of the funny lines and dark/light spots. A single hemi lamp will give even lighting with minimal shadows so you can more clearly see what's going in with your maps.